It depends on the ingredients.
Look for the active ingredient. It is often just caffeine in various forms (green tea, caffeine anhydrous), and vitamins- an expensive rip off.
However, some ingredients are unlabelled and illegal- you might be getting ephedrine in there which is not harmful in most cases- however think about how the fat burners work. They are supposed to have a thermogenic effect, which is basically raising your body temperature and making you sweat. In my opinion, that is not a sustainable loss because it is not burning fat, t is making you lose water weight.
Stimulants in general raise your blood pressure.
Some fat burners also have diuretics in them again just water weight.
Clenbuterol is dangerous, and can enlarge your heart. This is not something you would want, because it can lead to heart failure, your heart will not spring back into shape, it would be permanently "baggy" and stretched out.
If you look at the ads and read them critically, you will see that they always recommend an accompanying rigid diet and exercise program, and results are "not typical".